1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting wireless power, and more particularly, to method and apparatus for transmitting wireless power to multiple wireless power receivers.
2. Description of the Related Art
Recently, wireless or non-contact charging technologies have been developed, which are now widely used for a variety of electronic devices, such as wireless electric toothbrushes or wireless electric shavers.
Using wireless charging technology, which is based on wireless power transmission and reception, a battery of an electronic device, such as a mobile phone, may be automatically recharged if, for example, a user simply places the mobile phone on a charging pad without connecting a separate charging connector to the mobile phone.
Wireless charging technologies may be roughly classified into a coil-based electromagnetic induction scheme, a resonant scheme, and a Radio Frequency (RF)/microwave radiation scheme, which delivers electrical energy by converting it into microwaves.
Although the electromagnetic induction scheme has been used more often, recently, experiments using an RF/microwave radiation scheme have been successful. Thus, it is expected that in the near future, more types of electronic products will be recharged wirelessly.
The electromagnetic induction-based power transmission transmits power between a primary coil and a secondary coil. For example, an induced current occurs, when a magnet is moved around a coil. Using this principle, a transmitter generates a magnetic field, and in a receiver, a current is induced depending on a change in magnetic field, thereby producing energy. This power transmission method has excellent energy transmission efficiency.
As for the resonant scheme, power can be wirelessly transferred to an electronic device by using the Coupled Mode Theory, even though the electronic device is located several meters away from a charging device. The resonant scheme is based on a physics concept, wherein if a tuning fork rings, a nearby wine glass may also ring at the same frequency. However, the resonant scheme resonates electromagnetic waves containing electrical energy, instead of resonating sounds. The resonated electrical energy is directly delivered only to devices having the same resonant frequency, and any unused portion is reabsorbed as electromagnetic fields instead of being spread into the air. Thus, unlike other electromagnetic waves, the resonated electrical energy should not affect adjacent devices and a human body.
Although wireless charging schemes are garnering a great deal of attention and research, no standard has been proposed for the priority of wireless charging, a search for a wireless power transmitter and receiver, a selection of a communication frequency between the wireless power transmitter and receiver, an adjustment of wireless power, a selection of matching circuits, and a distribution of communication time for each wireless power receiver in one charging cycle. In particular, a standard is required for a wireless power transmitter to determine addition and removal of a wireless power receiver to and from a wireless power network managed by the wireless power transmitter.